1. Field of the Invention
The invention relates generally to ultrasonic flow meters. More particularly, the invention relates to liquid drainage systems for ultrasonic flow meters.
2. Background of the Technology
After hydrocarbons have been removed from the ground, the fluid stream (either in a liquid phase or a gaseous phase) is transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or during “custody transfer.” Even where custody transfer is not taking place, however, fluid flow rate measurement accuracy is desirable, and in these situations ultrasonic flow meters may be used.
An ultrasonic flow meter includes two or more transducer assemblies, each secured inside of a port in the body of the flow meter. The body of the flow meter may also be referred to as a spool piece. To contain the transported fluid within the flow meter, a connector is secured over the exterior end of each transducer port in the spool piece. Thus, the spool piece and end connectors create a pressure boundary and housing that contains the fluid flowing through the meter.
To measure fluid flow through the meter, a pair of transducer assemblies is positioned along the inner surface of the spool piece, such that each transducer assembly faces the other on opposite sides of the fluid flow through the bore of the spool piece. Each transducer assembly includes a piezoelectric element. When an alternating current is applied to the piezoelectric element of the first transducer assembly of the pair, the piezoelectric element responds by radiating an ultrasonic wave through the fluid flowing through the flow meter. When the wave is incident upon the piezoelectric element of the second transducer assembly of the pair, the second transducer assembly responds by generating an electric signal. Some time later, an alternating current is applied to the piezoelectric element of the second transducer assembly, and the second piezoelectric element responds by radiating an ultrasonic wave through the fluid in the flow meter to the first transducer assembly. When the wave is incident upon the piezoelectric element of the first transducer assembly, that transducer assembly responds by generating an electric signal. In this way, the transducer assemblies transmit and receive signals back-and-forth across the fluid stream.
Each transducer assembly is connected to a cable that extends through the end connector to the exterior of the spool piece and a remote location, such as an electronics base enclosure typically mounted to the outside of the spool piece. The cable carries the electric signal created by the piezoelectric element of the particular transducer assembly to an acquisition board positioned within the electronics base enclosure, where the signal may be processed and subsequently used to determine the fluid flow rate through the meter.
In most applications, it is common to have relatively small amounts of liquids in the gas stream flowing through the flow meter. Over time, some of the liquid may collect or accumulate within the transducer ports. However, the accumulation of an excessive amount liquid within the transducer port can detrimentally affect the transmission and/or receipt of signals by the transducer assembly disposed in the port. In particular, the accumulated liquid may cause the acoustic signal from the transducer assembly disposed in the port to acoustically short. In other words, the acoustic signal generated by the transducer assembly may pass into the liquid and/or meter body instead of passing exclusively through the fluid flowing through the meter body to the mating transducer assembly. As a result, the acoustic signal will not arrive at the mating transducer assembly, thereby causing flow measurement errors. In addition, accumulation of liquid in the transducer port may also cause shorting or corrosion of wires or other electrical components of the transducer assembly, potentially leading to failure of the transducer assembly.
To minimize the accumulation of fluids in transducer ports and associated problems, ultrasonic flow meters are typically mounted in specific orientations that allow accumulated liquid in the transducer ports to self drain, under the force of gravity, back into the main flow bore of the meter body. For example, ultrasonic flow meters employing transducer signal paths that cross the flow bore of the flow meter between a pair of transducer assemblies are typically recommended to be installed in horizontal piping (i.e., in-line with a horizontal section of piping) and positioned such that the transducer ports are oriented horizontally. The horizontal orientation of the transducer ports allows an liquid that has accumulated in the ports to self drain, under the force of gravity, back into the main flow bore of the meter body as long as the diameter of the port is uniform along its length or the diameter of the port increases moving towards the main flow bore of the meter body. As another example, ultrasonic flow meters employing transducer paths that bounce off the inner surface of the flow meter are typically recommended to be installed in horizontal piping (i.e., in-line with a horizontal section of piping) with the transducer port(s) positioned on the upper side of the flow meter and oriented vertically or at an angle less than 90° from vertical. The vertical or near vertical orientation of the transducer port(s) on the upper side of the meter body allows any liquid that has accumulated in the ports to self drain downward, under the force of gravity, back into the main flow bore of the meter body.
In the manner previously described, to reduce the accumulation of liquids in the transducer ports, flow meters and associated transducer ports are typically positioned and oriented in a specific manner. As a result, placement of the flow meter along a pipeline may be limited to those specific locations where the pipeline is horizontal. However, in some applications, it may not be possible, convenient, or cost effective to orient the flow meter as recommended. Accordingly, there remains a need in the art for ultrasonic flow meters that can be mounted in a greater variety of positions and orientations, while minimizing the accumulation of liquid in one or more of the transducer ports.